There exists a need for highly pure metal oxides and hydroxides, and particularly for highly pure oxides and hydroxides of transition metals, including the rare earth metals. One application for highly purified oxides and hydroxides is the growth of crystals having particular optical characteristics; for example, it has been found that crystals of very pure LiTaO.sub.3 and LiNbO.sub.3 are useful in laser systems to double the frequency of CO.sub.2 lasers. To grow LiTaO.sub.3 or LiNbO.sub.3 crystals, highly purified tantalum (V) oxide (Ta.sub.2 O.sub.5) and niobium (V) oxide (Nb.sub.2 O.sub.5) are required. Highly purified transition metal oxides are also useful in the electronic industry.
In cases where the metal is directly convertible to the oxide by burning, particularly in the case of the alkali metals and the alkaline earth metals, substantially pure metal oxides are obtainable by direct oxidation of the metals. In the case of many transition metals, including rare earth metals, direct oxidation is impractical, and the oxides are typically produced by hydrolysis of the corresponding halide. For example, tantalum (V) oxide and niobium (V) oxide are generally produced by hydrolysis of the corresponding chlorides: EQU 2TaCl.sub.5 +5H.sub.2 O.fwdarw.Ta.sub.2 O.sub.5 +10HCl EQU 2NbCl.sub.5 +5H.sub.2 O.fwdarw.Nb.sub.2 O.sub.5 +10HCl.
It has been found, however, that metal oxides, and metal hydroxides such as those currently produced by hydrolysis of chlorides, lack sufficient purity for many highly technical applications, such as growing optical grade crystals. In particular, commercial grade oxides of many metals contain a substantial amount of the halide as mixed oxide/halides. Apparently, the bond between the metals and the halide is sufficiently strong that complete removal of the halide by aqueous hydrolysis is not achieved. Furthermore, there do not exist practical means for completely separating the mixed oxide/halides from the metal oxides.